In a long term evolution (LTE) system or a system successive to the LTE system (also referred to as, for example, LTE-advanced (LTE-A), future radio access (FRA), or 4G), Device to Device (D2D) technology allowing user terminals to directly communicate with each other without using a radio base station has been studied (for example, see Non-Patent Document 1).
The D2D technology can reduce traffic between a user equipment and a base station and enable communication between user equipments even when a base station becomes incommunicable at the time of disaster or the like.
The D2D technology is roughly classified into D2D discovery (also referred to as D2D detection) for discovering another user terminal which is communicable and D2D communication (also referred to as D2D direct communication, direct communication between terminals) for allowing terminals to directly communicate with each other. Hereinafter, when the D2D communication and the D2D discovery are not to be particularly distinguished, the D2D communication and the D2D discovery are simply referred to as D2D. A signal which is transmitted and received by D2D is referred to as a D2D signal.